Abstract : An investigation into elemental asperity contact occurrences in rolling and sliding relative motion. Three steel balls were rotated between two superfinished steel flats thinly plated with a viscous polybutene lubricant. Contact occurrences were monitored by applying a voltage between a ball and flat and measuring the voltage drop across the same. Frictional forces in rolling and sliding were measured by a strain gauge, and a torque arm mounted on the upper flat. Rotating speeds were 0.25 and 0.69 R.P.M. The following factors were considered to describe the elemental interations between ball and flat surfaces in contact: (a) The (b) The microgeometry statistics of the contacting surfaces, defining the size, shape and distribution of individual asperities: (c) The (partial) elastohydrodynamic film formed in the Hertz contact area as it influences the average spacing between the contacting surfaces. On the basis of the observations, a tentative model of the asperity interactions is proposed. The experimental and theoretical methods used in studying individual asperity interactions are described. (Author)